The present invention is directed, in general, to a method for extruding food products, and more particularly is directed to such a method which utilizes a supercritical fluid such as carbon dioxide to simultaneously expand, flavor and reduce the viscosity of the food product, and to the apparatus for carrying out the method.
Extrusion cooking of foods is a well known process which is practiced on large commercial scales in countries throughout the world. Extrusion cooking generally involves the mixing of food products in an extruder under conditions of high temperature, high pressure and high shear, with the cooked product being extruded through an exit die. Often, the extruded food product is expanded, or puffed, by the release of steam after the product exits the die. However, in such prior procedures, the degree of puffing is dependent on a severe cooking process, which increases the barrel wear in the extruder, drastically increases the starch solubility of the extrudate, and limits the use of heat sensitive or shear sensitive ingredients, such as flavors or proteins, so that such ingredients must be added to the food product after the extrusion and cooking process, as by spraying the material onto the surface of the extrudate. Generally, in such prior processes, the extrudate has an appreciable moisture content to enable the product to flow through the extruder. This means that a part of the throughput of the extruder is water, and even though much of the water will flash off so as to expand the product at the outlet of the die, nevertheless the high moisture content requires a drying step in many cases. Furthermore, since the moisture content affects both the expansion of the product and its cell structure and texture, it is difficult to specify both expansion and texture independently in conventional processes.